ARD1 protein is a mammalian gene product homologous to a yeast Ard1p (Arrest defective 1 protein) acetyltransferase. Although two alternative splicing products of ARD1, ARD1(235) and ARD1(225), were reported in mouse, only ARD1(235) orthologue was reported in humans. Here we show that ARD1(225) is not expressed in humans, suggesting that factors regulating alternative splicing of ARD1 may have evolved differently between species. In human cells, hARD1(235) is shown to be present in both nucleus and cytoplasm. However, in mouse cells, mARD1(235) and mARD1(225) proteins are localized to the nucleus and cytoplasm, respectively. Moreover, during apoptosis, ARD1(235) and ARD1(225) isoforms are destabilized by different mechanisms in a species-specific manner and dependent on destabilizing reagents. These results indicate that ARD1(235) and ARD1(225) isoforms may have different activities and function in different subcellular compartments of mammalian cells. (c) 2006 Elsevier Inc. All rights reserved.